The present invention is concerned with a composition providing an aqueous bath for electrodeposition of gold and gold alloys on a substrate, and with a method of preparing the composition. The invention finds particular application, although it is not necessarily limited thereto, to the provision of an aqueous bath composition for the electrodeposition of a gold or gold alloy strike upon a stainless steel substrate.
Various compositions for electroplating gold or gold alloys are of course known in the art. For example, U.S. Pat. No. 3,598,706 shows a gold cyanide plating bath operable at low pH values. The plating bath employs gold cyanide compounds in which the gold content consists essentially of gold in its plus three valence state i.e., gold (III). A method of using the gold (III) salt employed is described as reacting a gold (III) salt such as auric oxide with an alkali metal cyanide such as potassium or sodium cyanide to form a mixture of the auric and auro (I) gold cyanide compounds. The auric gold species is then separated from the aurous gold species by dissolving the mixture and aging the solution to form an aurocyanide precipitate, which is then separated by filtration. The gold (III) cyanide crystals are obtained from the filtrate by crystallization. A preferred composition is cyanoauric (III) acid, citric acid and diammonium citrate.
U.S. Pat. No. 3,787,463 discloses a gold amine complex useful for electrodeposition of gold and its alloys. The gold amine complex is prepared by adding a polyamine to a solution of a gold (III) salt, for example, gold chloride, and then reacting the mixture with a sulfite containing or producing material and thereafter crystallizing the salt from the solution and recovering it by filtering and purifying by the usual methods. The preferred order of mixing the reactants is as described above; the polyamine may be ethylenediamine.
U.S. Pat. No. 3,458,542 discloses a combination complex of gold with an amine complex of another metal and gold in an aurous cyanide form, to provide a composition for the electrodeposition of the gold. The composition is prepared by dissolving a salt of the other metal in water, adding an amine and then adding an aqueous solution of gold dicyanide. The compound is recovered by precipitation from the solution and filtration.
One difficulty with such prior art compositions and methods is the relative complexity of the method of preparing the compositions which usually involves filtration, precipitation, etc.
Another problem is with which the electroplating art in general is concerned is the necessity for preparing the surface on which an initial layer or strike of metal is to be deposited. In particular, the deposition of an initial gold strike on a stainless steel surface usually requires preliminary pretreatment of the surface to prepare it to accept the gold strike.
It is accordingly an object of the present invention to provide a novel bath composition for an aqueous bath which may be prepared for electroplating a gold and/or gold alloy by a simple and efficient method without the necessity for crystallization, precipitation or filtration.
It is also an object to provide such a novel gold and/or gold alloy electroplating bath composition which is particularly suited to provide an initial strike of gold or gold alloy on an untreated stainless steel surface.
Another object is to provide a novel method for preparing such a composition.